The goal of this work is to obtain a full account of all the nerve cells providing input to, and all the nerve cells providing output from, a particular inhibitory neuron (the PV cell) in the cerebral cortex. In addition the location and number of synaptic connections will be delineated. Such a complete map of the connections to and from a neuron has not previously been attempted in the cerebral cortex. In order to do this we will need to use four different newly developed approaches that include both new optical and new electron microscopy strategies. It is my belief that it is likely that subtle changes in neural circuits may underlie abnormalities that are expressed outwardly as mental illness. Until we have methods to trace the connections at the finest level, such abnormalities will be inaccessible to scientific scrutiny. We have chosen as a first test a particularly interesting neuron that is likely to be located at pivotal regulatory point in the controlling neural wiring plasticity during development and perhaps disease. The particular goal will be to look at changes in the pattern of connections during development and in the present of certain gene deletions that are likely to be related to the way these cells function.